The Sending Unit
The sending unit is located in the fuel tank of the car. It consists
of a float, usually made of foam, connected to a thin, metal rod.
The end of the rod is mounted to a variable resistor. A resistor
is an electrical device that resists the flow of electricity. The
more resistance there is, the less current will flow. In a fuel
tank, the variable resistor consists of a strip of resistive material
connected on one side to the ground. A wiper connected to the gauge
slides along this strip of material, conducting the current from
the gauge to the resistor. If the wiper is close to the grounded
side of the strip, there is less resistive material in the path
of the current, so the resistance is small. If the wiper is at the
other end of the strip, there is more resistive material in the
current's path, so the resistance is large.

In the sending unit, the fuel has
to drop below a certain level before the float starts to drop.

When the float is near the top of the tank, the wiper
on the variable resistor rests close to the grounded (negative)
side, which means that the resistance is small and a relatively
large amount of current passes through the sending unit back to
the fuel gauge. As the level in the tank drops, the float sinks,
the wiper moves, the resistance increases and the amount of current
sent back to the gauge decreases.

This mechanism is one reason for the inaccuracy of
fuel gauges. You may have noticed how your gauge tends to stay on
full for quite a while after filling up. When your tank is full,
the float is at its maximum raised position -- its upward movement
is limited either by the rod it's connected to or by the top of
the tank. This means that the float is submerged, and it won't start
to sink until the fuel level drops to almost the bottom of the float.
The needle on the gauge won't start to move until the float starts
to sink.

Running on Empty
You may be surprised at how much fuel you actually have left when
the needle is on empty. To find out, check your owner's manual for
the exact volume of your fuel tank. Then, the next time your needle
shows empty, find the nearest gas station and fill 'er up. Subtract
the number of gallons it takes to fill your tank from the volume
stated in the owner's manual, and you'll know just how much farther
you can go when the gauge hits empty.

Something similar can happen when the float nears the bottom of
the tank. Often, the range of motion does not extend to the very
bottom, so the float can reach the bottom of its travel while there
is still fuel in the tank. This is why, on most cars, the needle
goes below empty and eventually stops moving while there is still
gas left in the tank.

Another possible cause of inaccuracy is the shape
of the fuel tanks. Fuel tanks on cars today are made from plastic,
molded to fit into very tight spaces on the cars. Often, the tank
may be shaped to fit around pieces of the car body or frame. This
means that when the float reaches the halfway point on the tank,
there may be more or less than half of the fuel left in the tank,
depending on its shape.

The Gauge
The gauge is also a simple device. The current from the sender passes
through a resistor that either wraps around or is located near a
bimetallic strip. The bimetallic strip is hooked up to the needle
of the gauge through a linkage.

The bimetallic strip is a piece of metal made by laminating
two different types of metal together. The metals that make up the
strip expand and contract when they are heated or cooled. Each type
of metal has its own particular rate of expansion. The two metals
that make up the strip are chosen so that the rates of expansion
and contraction are different.

When the strip is heated, one metal expands less than
the other, so the strip curves, with the metal that expands more
on the outside. This bending action is what moves the needle.

Some newer cars, instead of sending the current directly
to the gauge, use a microprocessor that reads the output of the
resistor and communicates with the dashboard. These systems actually
help improve the accuracy of the gauge. Let's take a look at one
of these systems.

Microprocessor-controlled Gauges
Some newer cars have a microprocessor that reads the variable resistor
in the tank and communicates that reading to another microprocessor
in the dashboard. Carmakers can tinker with the gauge movement a
little -- they can compensate for the shape of the tank by comparing
the float position to a calibration curve. This curve correlates
the position of the float with the volume of fuel left in the tank.
This allows the gauge to read more accurately, especially in cars
with complicated gas-tank shapes.

Systems like this can also trigger a fuel light that
signals when fuel is getting low. Most of these lights come on while
there are still a couple of gallons of gas left in the tank, giving
you plenty of time to stop for fuel.

The microprocessor can also provide some damping to
the needle movement. When you go around a turn, or up a hill, the
fuel can slosh to one side of the tank and quickly change the float
position. If the needle were to respond quickly to all of these
changes, it would be bouncing all over the place. Instead, software
calculates a moving average of the last several readings of the
float position. This means that changes in needle position occur
more slowly. You may have noticed this when filling up your car
-- you'll finish filling the tank long before the needle reaches
full.

While fuel gauges are far from exact, they err
on the conservative side.